With appearance of services associated with various types of digital contents such as broadcastings, movies, music and UCC (User Created Content), demands for content recognition technology for protecting and utilizing the contents are drastically being increased.
A content recognition system receives content signals of an input content and recognizes the input content by searching a previously created feature index database. The content recognition system can be applied to various fields such as monitoring of broadcasting advertisements and music, file filtering in a file sharing service and the like. In order to be used in the above-described fields, the content recognition system needs to efficiently store a large amount of contents in a database, to rapidly retrieve the contents from the database and to accurately recognize various content signals transformed via compression, filtering and the like. In particular, in the monitoring of the broadcasting and the file filtering based on a mass content feature database, not only accurate recognition but also size-reduction of the database and rapid content recognition for real-time processing are very important factors.
There exist many content recognition systems. A first example is a system in which features are extracted from feature points (referred to as landmarks) of an audio content and stored in a database. A second example is a system in which features extracted from image contents, e.g., fingerprints, are sub-sampled and sequentially indexed.
Since an amount of the features to be stored in the database are increased along with an increase of an amount of the contents, there is a serious need for a content recognition system capable of reducing the index database in size.
However, since local features of the audio content, i.e., the “landmarks”, are used as they are in the first example and the features are simply sub-sampled and indexed sequentially in the second example, the size of the database becomes too large, which results in a significantly long data retrieval time from the database.